Embodiments relate to induction furnaces for heating a workpiece in an inert atmosphere or vacuum. In particular, embodiments employ various improvements to induction furnaces that allow less complex and less costly manufacture.
Conventional induction furnaces include an induction heating system and a chamber that contains a susceptor that is susceptible to induction heating. An electromagnetic coil sits outside the susceptor and receives high frequency alternating current from a power supply. The resulting alternating electromagnetic field heats the susceptor rapidly. The workpiece to be heated is placed in proximity to and generally within the susceptor so that when the susceptor is inductively heated by the induction heating system, the heat is transferred to the workpiece through radiation and/or conduction and convection. In a prior art system, a mating two-piece quartz chamber is employed as an insulation system. The top, bell-shaped piece of the chamber is fixed, while the bottom, bowl-shaped piece moves up and down with the stage on which the workpiece is placed. The chamber walls sit between the coil and the susceptor to substantially reduce heat leakage and improve heating efficiency. Thus, the mating two-piece quartz chamber arrangement provides insulation completely around the susceptor. While the quartz chamber excels at providing heat leakage insulation, it is somewhat costly to manufacture and somewhat fragile in nature. Thus, an alternative structure is desirable to decrease cost and improve durability.
An induction heating furnace employing the two-piece insulator described above is shown, for example, in FIGS. 1 and 2. The induction furnace 100 includes an induction heating system and a chamber 104 that comprises a quartz cylinder 110, a first cover 112 for sealing one end of the cylinder, and a second cover 114 for sealing the second end of the cylinder. The induction heating system includes a coil 120 and a power supply (not shown) that provides an alternating current that flows through the coil 120 during a heating cycle. The coil 120 is wound to form a cylindrical shape within the chamber 104, as shown in FIG. 1.
Contained within the chamber 104 is a susceptor 130 that is susceptible to induction heating. That is, when an alternating current flows through the coil 120, an alternating magnetic field is generated that induces eddy currents and other effects in the susceptor 130 that cause the susceptor 130 to heat. The thermal energy that radiates from the susceptor 130 is used to heat a workpiece 190. The susceptor 130 is shown as being cylindrical, but other shapes can be used. The susceptor 130 is made of any material susceptible to induction heating, such as, for example, graphite, molybdenum, steel, and tungsten. The susceptor 130 can be arranged within a thermal insulator 140 disposed substantially between the susceptor 130 and the inner walls of cylinder 110 in the chamber 104. The insulator 140 can be a cylindrical body 141 made from, for example, fused quartz. As shown in FIG. 1, insulator 140 can include additional fused quartz containers, such as a second fused quartz container 151.
The fused quartz container 141 can comprise two pieces: a first piece 142; and a second piece 144. The first piece 142 is connected to the first cover 112 of quartz cylinder 110 and the second piece 144 is connected to the second cover 114 of the quartz cylinder 110. Ceramic posts 161 can connect the first piece 142 to the first cover 112 and additional ceramic posts 162 can connect the second piece 144 to the second cover 114. A slight gap 164 between the first piece 142 and the second piece 144, such as of about 0.10 inches wide, can be employed to allow air to be evacuated from within the containers 141.
Similarly, the second fused quartz container 151 can comprise two pieces: a first piece 152; and a second piece 154. The first piece 152 is connected to the first piece 142 of the first container 141 and the second piece 154 is connected to the second piece 144 of the first container 141. As with the first container 141, a slight gap 166 between the first piece 152 and the second piece 154, such as of about 0.10 inches wide, can be employed to allow air to be evacuated from within the containers 141, 151. Preferably, as shown in FIG. 1, the gaps 164, 166 are not aligned to reduce heat leakage.
The susceptor 130 can also comprise two pieces: a first piece 132; and a second piece 134. The first piece 132 of the susceptor 130 is connected to the first piece 152 of the second container 151, and the second piece 134 of the susceptor 130 is connected to the second piece 154 of the second container 151. A tray 155 for supporting the workpiece 190 to be heated is connected to the second piece 134 of the susceptor 130. Although the susceptor 130 is shown as having closed ends, this need not be the case. For example, the susceptor 130 can be in the form of a tube that is open at both ends or, for example, it can comprise one or more susceptor sheets. At least one of the first and second covers 112, 114 is releasably connected to the quartz cylinder 110 so that the cover can be easily removed, thus providing a convenient mechanism for loading and unloading workpiece 190, as shown in FIG. 2.
The induction furnace 100 also includes a vacuum pump 170 for creating a vacuum within the chamber 104 and a cooling system 172 for cooling the chamber 104 after the workpiece has been heated as desired. The cooling system 172 can include a heat exchanger 174 and a blower 176. Hot air within the chamber 104 is drawn into the heat exchanger 174 and cooler air is blown back into the chamber 104 by the blower 174. To protect the vacuum pump 170, a gate or knife valve 178 can be interposed between the pump 170 and the chamber 104. The valve 178 shuts upon the beginning of the cooling cycle, thereby protecting pump 170.
Embodiments contemplate a new enclosure to further protect the surroundings from the extreme temperatures generated within the furnace while reducing costs and increasing efficiency. An annular enclosure is preferred, with its longitudinal axis normal to the ground or floor. Top and bottom covers are preferably employed to seal off the enclosure, though the bottom cover is preferably movable along the longitudinal axis of the enclosure to accommodate movement of the workpiece stage. Embodiments provide for cooling of the annular enclosure by circulation of water within the annular walls. Thus, a gap is formed between inner and outer walls of the annular enclosure and cooling water is pumped into the gap. Vanes are preferably formed in the gap to induce helical flow about the longitudinal axis of the enclosure, enhancing the cooling efficiency of the apparatus. In embodiments, a top cover seals the top end of the cylinder, and a bottom cover seals the bottom end of the cylinder, one or both of which can also be water cooled. The induction heating system includes a coil connected to a power supply. The coil surrounds the quartz cylinder, but lies within the steel cylinder. The susceptor lies within the fused quartz cylinder, as does the workpiece stage.
Advantageously, the susceptor comprises two pieces: an upper piece and a lower piece. The upper piece is connected to the top cover of the stainless steel cylinder and the lower piece is connected to the bottom cover and the stage. The bottom cover is releasably connected to the upper piece of the cylinder so that it can be easily removed, thus providing a convenient mechanism for loading and unloading the workpiece.
Additionally, embodiments employ susceptible materials for the wall of the outer housing of the furnace by arranging a distance between an inner wall and the induction coil within, as well as special selection of AC frequencies, to prevent electromagnetic field coupling of the wall. For example, embodiments can employ stainless steel, which is much less costly than quartz.
Another heat control arrangement involves the manner of construction of the coil. Preferably, the coil is hollow to allow cooling water to flow therein. Since the coil must conduct electricity, the coil must be made from a conductor, such as a conductive metal. Thus, the coil is preferably made from metal tubing, such as copper tubing.
A further heat control arrangement employed in embodiments is the formation of at least one insulative air gap at at least one end of the susceptor. Such an air gap is preferably formed between two discs separated by a spacer. While many materials could be used, graphite discs are preferred in embodiments. Additionally, ceramic or graphite rings are preferred as spacers between the discs. Graphite and ceramic materials are particularly hardy in the type of environment to which these parts are exposed and so enhance the life of the parts when used.
Still another heat control arrangement used in embodiments is the inclusion of one or more infrared radiation reflectors. In particular, a reflector can be placed at an end of the susceptor to reduce heat leakage from the end of the susceptor. This is particularly useful when a cylindrical fused quartz insulator is employed in the chamber, since the open ends of the quartz insulator do not provide insulation. Embodiments employ a disc at each end of the susceptor, preferably made from molybdenum or a similarly robust and infrared radiation reflective substance. Preferably, embodiments use at least one such reflector at each end of the susceptor: one can be mounted on the support of the susceptor, and another can be mounted on under the work piece stage, for example.
By special selection of the frequencies employed in the induction coil, a dual heating effect can be achieved in embodiments. For example, frequencies in a range of from about 8 kHz to about 10 kHz penetrate the insulation and couple into the susceptor material while also coupling into a conductive object being treated in the susceptor. The coupling into the treated object provides direct induction heating of the object in addition to radiational heating from the susceptor walls, increasing the efficiency of the furnace.
The above and other features of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying drawings.